Gas hydrates are clathrates (inclusion compounds) of small molecules in a lattice of water molecules. In the petroleum industry, natural gas and petroleum fluids contain a variety of these small molecules, which can form gas hydrates. They include hydrocarbons such as methane, ethane, propane, isobutane as well as nitrogen, carbon dioxide and hydrogen sulphide. Larger hydrocarbons such as n-butane, neopentane, ethylene, cyclopentane, cyclohexane and benzene are also hydrate-forming components. When these hydrate-forming components are present with water at elevated pressures and reduced temperatures, the mixture tends to form gas hydrate crystals. For example, ethane at a pressure of 1 MPa forms hydrates only below 4° C., whereas at 3 MPa gas hydrates can only form below 14° C. These temperatures and pressures suited to hydrate formation are typical operating environments where petroleum fluids are produced and transported and in drilling, completion or fracturing operations in the oil and gas industry.
If gas hydrates are allowed to form inside a pipe containing natural gas and/or other petroleum fluids, they can eventually block the pipe. The hydrate blockage can lead to a shutdown in production and significant financial loss. The oil and gas industry therefore uses various means to prevent the formation of hydrate blockages in pipelines. These include heating the pipe, reducing the pressure, removing the water and adding thermodynamic inhibitors (antifreezes) such as methanol and ethylene glycols, which act as melting point depressants. Each of these methods is costly to implement and maintain. The most common method used today is the addition of antifreezes. However, these antifreezes have to be added at high concentrations, typically 10-60% by weight of the water present, in order to be effective. Recovery of the antifreeze is also often required and is a costly procedure.
An alternative to the above methods is to control the gas hydrate formation process using nucleation and crystal growth inhibitors. These types of chemicals are widely known and used in other industrial processes. The advantage of using these chemicals to control gas hydrate formation is that they can be used at concentrations of 0.01 to 3%, i.e. much lower than concentrations typically used for antifreezes. Thus, these chemicals are often called low dosage hydrate inhibitors (LDHIs).
Gas hydrate nucleation inhibitors are called kinetic hydrate inhibitors (KHIs). Examples of KHIs include polyvinylpyrrolidone, copolymers of vinyl pyrrolidinone (e.g. with alpha-olefins, vinyl caprolactam or dimethylaminoethyl methacrylate), polymers containing pyrrolidinocarbonyl aspartate groups, polyesteramides and polyvinyllactams. KHI polymers are often expensive, therefore a lower concentration of KHI polymer (perhaps 40-60% as much) is often used with the addition of a cheaper synergist to improve the performance and lower the overall cost. A commonly used KHI synergist is the quaternary ammonium salt, tetrabutylammonium bromide (TBAB).
Some kinetic hydrate inhibitor polymers cannot be used on some oil/gas fields because they have a cloud point (or lower critical solution temperature) in the produced aqueous fluid below the temperature where the polymer would be injected, e.g. at the wellhead. This would cause the polymer to deposit near the injection point rendering it ineffective for the job for which it was designed. It could also cause a restriction in the conduit near the injection point. It would therefore be advantageous if alternative additives could be found.
Besides KHIs, there is another class of LDHIs called anti-agglomerants (AAs). AAs do not inhibit the formation of gas hydrates to the same level as KHIs, rather their primary activity is in preventing the agglomeration and deposition of hydrate crystals. A hydrocarbon phase provides a transport medium for the hydrates which are referred to as hydrate slurries so that the overall viscosity of the medium is kept low and can be transported along the pipeline. As such, the hydrate crystals formed in the water-droplets are prevented from agglomerating into a larger crystalline mass. Chemicals acting as anti-agglomerate hydrate inhibitors are typically quaternary ammonium or phosphonium salts, such as tributylhexadecylphosphonium bromide and tributylhexadecylammonium bromide.
Unfortunately, such compounds have undesirable levels of toxicity, are poorly biodegradable and don't function well in water with relatively low salt concentrations (such as some areas of the North Sea).
Due to the above-mentioned problems relating to cost, performance and environmental impact, a need exists for alternative compounds for inhibiting and controlling the formation of gas hydrates in connection with hydrocarbon production, storage and transportation including production, drilling, completion, fracturing, stimulation and injection and reinjection operations.